The invention disclosed herein relates generally to a method and apparatus for producing a digital representation of a photodiode signal, and more particularly to a switched capacitor successive approximation analog to digital conversion method and circuitry for directly producing a digital signal representative of the charge on a photodiode.
It is well known to use a reverse biased semiconductor p-n junction to convert light intensity into an electrical signal. This characteristic of a photodiode has long been used in a variety of light sensing applications. It has also become well known to employ arrays of photodiodes in optical recognition applications. The more elementary applications require only that the output signal of a photodiode element indicate whether light intensity is above or below a predetermined level. As an example, many optical character recognition systems only require information as to the presence or absence of a marking at a spot being read.
More recently, increasing numbers of image sensing applications have emerged requiring gray scale representations of a detected image. Notable examples may be found in the fields of robotics and factory automation, as well as in a rapidly expanding variety of other applications.
Since the majority of such applications utilize digital signal processing, it is necessary to obtain digital representations of the photodiode signals. The signal produced by a photodiode is an electric charge, or more specifically, a change in electric charge. The most common conventional approach to digitization is to convert the charge signal to an analog voltage level, and then perform an analog to digital conversion of the voltage level to produce a digital signal. Although the technology for accomplishing these operations is well developed and capable of producing highly satisfactory results, elimination of any of the signal processing steps offers the potential for greater simplicity, lower manufacturing cost, smaller size and greater accuracy. Alternatively, semiconductor area and other resources released by elimination of unnecessary operations may be used for more sophisticated signal processing and peripheral functions.
The present invention achieves significant advances in minimizing the size and expense of a photodetector device with digital gray scale readout capability by directly converting the changes in charge on reverse biased photodiodes in a diode array to digital representations, while retaining good performance capabilities. A circuit in accordance with the applicant's invention is well suited for fabrication with standard CMOS integrated circuit designs which provide for low power consumption and low cost.